The invention is an electronic circuit that holds the gain of a fiber optic (FO) link constant despite optical parameter variations that would otherwise cause gain variations.
FO links are used to carry RF signals between two distant locations. An externally modulated fiber optic link 20 is shown in FIG. 1. Light is generated by the continuous wave (CW) laser 22 and injected into the input fiber waveguide 24. The light is carried to the fiber optic modulator 26, which acts as a gate and varies the power in accordance with the RF input voltage (RF SIGNAL IN). The modulated light is carried by the output fiber 28 to the photodetector 30, where it is detected. A bias circuit 32 applies a DC bias to the photodetector 30. The detection process converts the modulated light into a current proportional to the modulating voltage, as shown in FIG. 2.
FO links, especially externally modulated links, suffer from RF gain fluctuations which cause problems for certain high performance applications where stability is important. The fluctuations can arise from laser power variations or optical loss variations caused by connectors or fiber bending. For certain applications it is very important that the gain, i.e., the ratio of the output signal to the input signal, stay constant. The gain of the link 20 is sensitive to the laser power and to the optical losses in the link. However, the gain and the average detected optical power vary in direct proportion, and this may be used, in accordance with the invention, to keep the gain constant.
There has been some previous attempts to stabilize the gain by controlling a voltage variable attenuator with the average detected current, but the attenuator was strictly an RF type and the control signal was taken from the detector side. Such an arrangement is subject to drift and low accuracy due to attenuator control nonlinearities.
Others have proposed to use an average photodetector current as a reference in a feed forward, open loop gain control circuit. The disadvantage with such gain control circuits is that they can easily lose accuracy due to tolerances, temperature drifts and component aging.
It is therefore an object of this invention to provide a gain control circuit for a FO link that compensates for component temperature drifts, aging and nonlinearities.
A further object is to provide a gain control circuit for a FO link which provides a noise reduction function.